Multiactive electrophotographic element

ABSTRACT

An improved reusable multiactive electrophotographic element has a charge-transport layer comprising a triarylamine charge-transport material in a binder comprising a polyester containing recurring units having the structure   &lt;IMAGE&gt;

FIELD OF THE INVENTION

This invention relates to multiactive electrophotographic elements,i.e., elements containing a charge-generation layer and acharge-transport layer. More particularly, the invention relates to suchelements which are reusable and contain a triarylamine charge-transportmaterial in the charge-transport layer.

BACKGROUND

In electrophotography an image comprising a pattern of electrostaticpotential (also referred to as an electrostatic latent image), is formedon a surface of an electrophotographic element comprising at least aninsulative photoconductive layer and an electrically conductivesubstrate. The electrostatic latent image is usually formed by imagewiseradiation-induced discharge of a uniform potential previously formed onthe surface. Typically, the electrostatic latent image is then developedinto a toner image by bringing an electrographic developer into contactwith the latent image. If desired, the latent image can be transferredto another surface before development.

In latent image formation the imagewise discharge is brought about bythe radiation-induced creation of electron/hole pairs, which aregenerated by a material (often referred to as a charge-generationmaterial) in the electrophotographic element in response to exposure tothe imagewise actinic radiation. Depending upon the polarity of theinitially uniform electrostatic potential and the types of materialsincluded in the electrophotographic element, either the holes or theelectrons that have been generated, migrate toward the charged surfaceof the element in the exposed areas and thereby cause the imagewisedischarge of the initial potential. What remains is a non-uniformpotential constituting the electrostatic latent image.

Such elements may contain material which facilitates the migration ofgenerated charge toward the oppositely charged surface in imagewiseexposed areas in order to cause imagewise discharge. Such material isoften referred to as a charge-transport material.

One type of well-known charge-transport material comprises atriarylamine. The term, "triarylamine," as used herein is intended tomean any chemical compound containing at least one nitrogen atom that isbonded by at least three single bonds directly to aromatic rings or ringsystems. The aromatic rings or ring systems can be unsubstituted or canbe further bonded to any number and any types of substituents. Suchtriarylamines are well known in the art of electrophotography to be verycapable of accepting and transporting charges generated by acharge-generation material.

Among the various known types of electrophotographic elements are thosegenerally referred to as multiactive elements (also sometimes calledmultilayer or multi-active-layer elements). Multiactive elements are sonamed, because they contain at least two active layers, at least one ofwhich is capable of generating charge in response to exposure to actinicradiation and is referred to as a charge-generation layer (hereinaftersometimes alternatively referred to as a CGL), and at least one of whichis capable of accepting and transporting charges generated by thecharge-generation layer and is referred to as a charge-transport layer(hereinafter sometimes alternatively referred to as a CTL). Suchelements typically comprise at least an electrically conductive layer, aCGL, and a CTL. Either the CGL or the CTL is in electrical contact withboth the electrically conductive layer and the remaining CGL or CTL. TheCGL comprises at least a charge-generation material; the CTL comprisesat least a charge-transport material; and either or both layers mayadditionally comprise a film-forming polymeric binder.

Among the known multiactive electrophotographic elements, are thosewhich are particularly designed to be reusable and to be sensitive toimagewise exposing radiation falling within the visible and/or infraredregions of the electromagnetic spectrum. Reusable elements are thosethat can be practically utilized through a plurality (preferably a largenumber) of cycles of uniform charging, imagewise exposing, optionaldevelopment and/or transfer of electrostatic latent image or tonerimage, and erasure of remaining charge, without unacceptable changes intheir performance. Visible and/or infrared radiation-sensitive elementsare those that contain a charge-generation material which generatescharge in response to exposure to visible and/or infrared radiation.Many such elements are well known in the art.

For example, some reusable multiactive electrophotographic elementswhich are designed to be sensitive to visible radiation are described inU.S. Pat. Nos. 4,578,334 and 4,719,163, and some reusable multiactiveelectrophotographic elements which are designed to be sensitive toinfrared radiation are described in U.S. Pat. Nos. 4,666,802 and4,701,396.

Many known reusable multiactive electrophotographic elements sensitiveto visible or infrared radiation also employ triarylaminecharge-transport materials in their CTL. In those elements thetriarylamine is dispersed or dissolved in a film-forming polymericbinder that forms the CTL. Such elements are described, for example, inthe four U.S. patents noted above. Those patents teach many polymers ashaving utility as film-forming binders for CTL's. Among the manypolymers so described, are polycarbonates, such aspoly[2,2-bis(4-hydroxyphenyl)propane carbonate] (commonly referred to asbisphenol A polycarbonate), and polyesters. Elements containing suchcomponents fairly adequately perform their intended functions, and, inthe case of the elements described in the four U.S. patents noted above,have some very important advantages over other known elements. However,it has been recognized (e.g., in U.S. Pat. Nos. 4,840,860 and 4,840,861)that there are some significant drawbacks associated with such elements.

For example, if the CTL comprises a triarylamine in a bisphenol Apolycarbonate film, a significant problem may arise. The problem canoccur when the CTL has been adventitiously exposed to ultravioletradiation (i.e., radiation of a wavelength less than about 400nanometers, which, for example, forms a significant portion of theradiation emitted by typical fluorescent room lighting). This can occur,for example, when the electrophotographic element is incorporated in acopier apparatus and is exposed to typical room illumination duringmaintenance or repair of the copier's internal components. The problem,which has been referred to as a UV-fogging problem, is manifested as abuildup of residual potential within the electrophotographic elementover time as the element is exercised through its normal cycles ofelectrophotographic operation after having been adventitiously exposedto ultraviolet radiation.

For example, in normal cycles of operation such an element might beinitially uniformly charged to a potential of about -500 volts, and itmight be intended that the element should then discharge, in areas ofmaximum exposure to normal imagewise actinic visible or infraredexposing radiation, to a potential of about -100 volts, in order to formthe intended electrostatic latent image. However, if theelectrophotographic element has been adventitiously exposed toultraviolet radiation, there will be a buildup of residual potentialthat will not be erased by normal methods of erasing residual chargeduring normal electrophotographic operation. For example, after about500 cycles of operation, the unerasable residual potential may be asmuch as -200 to -300 volts, and the element will no longer be capable ofbeing discharged to the desired -100 volts. This results in a latentimage being formed during normal operation, that constitutes aninaccurate record of the image intended to be represented. In effect,the element has become no longer reusable, after only 500 cycles ofoperation.

While the mechanism of this UV-fogging problem is not presentlyunderstood, U.S. Pat. Nos. 4,840,860 and 4,840,861 theorize that theproblem may be caused by a chemical change in the triarylaminecharge-transport material, induced by absorption of ultravioletradiation. This is evidenced by an observed color change in the CTLafter exposure to ultraviolet radiation. It would be desirable to beable to avoid or minimize this UV-fogging problem.

On the other hand, U.S. Pat. Nos. 4,840,860 and 4,840,861 haverecognized that, if the electrophotographic element comprises a CTL,wherein the triarylamine is contained in a binder film of one of certainpolyesters, the UV-fogging problem does not arise. Those patentstheorize that this may be because the polyester absorbs more ultravioletradiation than does a bisphenol A polycarbonate, and thus prevents someof the ultraviolet radiation from being absorbed by the triarylamine inamounts significant enough to cause the chemical change that leads tothe UV-fogging problem, and/or the polyester or some complex of thepolyester with the triarylamine may otherwise quench or prevent theUV-induced chemical change from occurring.

Unfortunately, such elements having a polyester as their CTL binderexhibit another drawback recognized in U.S. Pat. Nos. 4,840,860 and4,840,861; namely, they have significantly lower sensitivity to actinicvisible or infrared radiation (sometimes referred to as lower speed)than do elements that utilize bisphenol A polycarbonate as their CTLbinder. For example, in some cases the exposure to actinic radiationnecessary for discharging the initial uniform electrostatic potentialfrom -500 to -100 volts (sometimes referred to as the 100-volt speed),is about 75 percent more when a polyester is the CTL binder, comparedwith when bisphenol A polycarbonate is the CTL binder. This is a verysignificant difference in terms of high speed copiers; i.e., the copierusing polycarbonate as the CTL binder can make more than 5 exposures inthe same time it takes the copier with the polyester CTL binder to make3 exposures. It would, of course, be desirable to retain this speedadvantage of the polycarbonate.

It thus became evident that there was a need for a reusable visibleand/or infrared-sensitive electrophotographic element that avoids orminimizes the UV-fogging problem of elements utilizing a polycarbonateCTL binder, while at the same time avoiding or minimizing the speed lossinherent in elements utilizing certain polyester CTL binders.

The inventions described in U.S. Pat. Nos. 4,840,860 and 4,840,861 meetthis need by providing electrophotographic elements wherein the CTLcomprises binders that are mixtures of certain polycarbonates withcertain polyesters. It was found that such mixtures synergisticallyprovide most of the UV-fogging avoidance of certain polyesters whileretaining most of the speed advantage of certain polycarbonates.

While those inventions provide great benefit, there are other drawbacksassociated with them. Namely, the need to employ a mixture of two binderpolymers in the same layer, rather than just a single binder polymer,requires that one be concerned with the compatibility of the polymerswith each other and with charge-transport agents and any other materialsdesired to be included in a CTL. Any incompatabilities between suchmaterials can result in phase separations during preparation or use ofthe electrophotographic element. Such phase separations can cause pooreror nonuniform electrical performance in the element and can causeundesirable scatter or absorption of actinic radiation during imagewiseexposure, resulting in poorer image accuracy and resolution. The risk ofthis occurring is inherently greater when two polymers are employedinstead of one.

Therefore, a need still existed for a binder polymer for atriarylamine-containing CTL, which would avoid the UV-fogging problemwhile enabling the electrophotographic element to exhibit betterelectrophotographic speed than is afforded by other polymers known to beuseful for UV-fogging avoidance, and which would accomplish this withouta need to be combined in a mixture with other polymers. The presentinvention satisfies this need.

SUMMARY OF THE INVENTION

It has been unexpectedly found that the UV-fogging problem associatedwith polycarbonate CTL binder canbe avoided if a certain polyester isemployed as the CTL binder. It has also been unexpectedly found thatsuch polyester CTL binder enables an electrophotographic element toexhibit better electrophotographic speed than do other polymeric CTLbinders that are known to avoid UV-fogging, even if it is not combinedin a mixture with other polymeric binders.

Thus, the invention provides an electrophotographic element comprising:an electrically conductive support; a charge-generation layer sensitiveto visible or infrared radiation; and a charge-transport layercontaining a triarylamine charge-transport material. The elementadditionally contains the improvement wherein the charge-transport layercomprises a polyester containing recurring units having the structure##STR2##

DESCRIPTION OF PREFERRED EMBODIMENTS

As previously defined, the invention pertains to any reusablemultiactive electrophotographic element designed to be sensitive tovisible and/or infrared radiation and containing any triarylaminecharge-transport material in a polymeric CTL. Elements of that type andtheir preparation and use are well known in the art ofelectrophotography. For detailed description of such elements and theirpreparation and use, see, for example, U.S. Pat. Nos. 3,041,166;3,165,405; 3,394,001; 3,679,405; 3,725,058; 4,175,960; 4,284,699;4,578,334; 4,666,802; 4,701,396; and 4,719,163, the disclosures of whichare hereby incorporated herein by reference. The only essentialdifference between such wellLknown elements and elements of the presentinvention is in the present use of a particular polyester binder in theCTL.

Although the invention is applicable when any triarylamine serves as acharge-transport material in the CTL, in a particularly preferredembodiment of the invention, the CTL contains the charge-transportmaterial, 1,1-bis[4-(di-4-tolylamino)phenyl]-3-phenylpropane.

Of course, multiactive electrophotographic elements of the invention cancontain any of the optional additional layers and components known to beuseful in reusable multiactive electrophotographic elements in general,such as, e.g., subbing layers, overcoat layers, barrier layers,screening layers, additional binders, leveling agents, surfactants,plasticizers, sensitizers, and release agents.

Structure (I), illustrated above to describe recurring units containedin the polymer employed in the charge-transport layer of an element inaccordance with the invention, is intended to encompass the followingalternative isomeric Structures (I-A) and (I-B): ##STR3##

The polymer in the charge-transport layer of an element of the inventioncan contain recurring units having Structure (I-A), recurring unitshaving Structure (I-B), or recurring units having Structure (I-A) andrecurring units having Structure (I-B). All three alternatives serve thepurposes of the invention well. In a preferred embodiment of theinvention the polymer contains recurring units having Structure (I-A).

The polyesters having recurring units of structure (I) employed inelements of this invention can be prepared by methods generally known tobe useful for polyester syntheses, e.g., by condensation of appropriatediacids (or their esters or salts) with appropriate diols. For example,appropriate diacid salts are terephthaloyl chloride and isophthaloylchloride, which are readily commercially available, e.g., from theEastman Kodak Co., U.S.A. An appropriate diol is tetramethylbisphenol A,which can be prepared by condensation of 2,6-dimethylphenol withacetone. Further details of preparations of the diol and an appropriatepolyester are presented in Preparations 1 and 2, below. Polyestershaving recurring units of Structure (I), that are useful in accordancewith the invention, have weight average molecular weights within therange of from 10,000 to 200,000.

The following preparations and example are presented to furtherillustrate a preferred electrophotographic element of the invention andto compare it's properties and performance to those of elements outsidethe scope of the invention.

A polyester containing recurring units having Structure (I) wassynthesized as described in Preparations 1 and 2, below.

Preparation 1: Tetramethylbisphenol A

In a 1-liter 3-necked round-bottom flask equipped with a condenser,stirrer and HCl gas inlet tube, was placed 244 g (2.0 mol) of2,6-dimethylphenol and 116 g (2.0 mol) of reagent grade acetone. HCl gaswas then bubbled into the reaction mixture for approximately 5 hours(i.e., until the mixture was saturated with HCl). The reaction mixturewas stirred at room temperature for 24 hours, and the solids werefiltered and washed twice with 1 liter of hexanes, followed by 1 literof distilled water, then again with hexanes. The crude product wasrecrystallized from 1.5 liters of 80% aqueous methanol, collected, anddried in a vacuum oven at 50° C. for 24 hours to give 185 g (65%) of thedesired product as white crystals.

Melting point=164° C.

Elemental Analysis: calculated for C₁₉ H₂₄ O₂ :

80.2% C, 8.5% H; found: 80.2% C, 8.5% H.

Preparation 2: Poly(tetramethylbisphenol A terephthalate). Structure(I-A)

To a stirred mixture of tetramethylbisphenol A (28.44 g, 0.10 mol) andtriethylamine (22.3 g, 0.22 mol) in methylene chloride (100 ml) at 10°C. was added a solution of tezephthaloyl chloride (20.3 g, 0.10 mol) inmethylene chloride (70 ml). After addition, the temperature was allowedto rise to room temperature, and the solution was stirred under nitrogenfor 4 hours, during which triethylamine hydrochloride precipitated in agelatinous form and the solution became viscous. The solution was thenfiltered and washed with dilute hydrochloric acid, 2% (100 ml) followedby water (3×200 ml). The solution was then poured into methanol withvigorous stirring, and a white fibrous polymer, the desired product,precipitated. The isolated polymer was dried in a vacuum oven at 40° C.for 24 hours to give a 97% yield (40 g).

Weight average molecular weight =48,100.

Number average molecular weight =20,800.

(Molecular weights were determined by gel permeation chromatographybased on polystyrene equivalents.)

Glass transition temperature (by differential scanning calorimetry)=234°C.

EXAMPLE 1

An electrophotographic element of the invention was prepared as follows.

A conductive support was utilized, comprising a 178 micrometer thicknessof poly(ethylene terephthalate) film having vacuum-deposited thereon athin conductive layer of nickel.

An adhesive layer was coated onto the nickel surface of the conductivesupport from a solution of 4.8 g of poly(acrylonitrile-co-vinylidenechloride) (17:83 molar ratio) in 1.2 kg of methyl ethyl ketone solventand dried. Coverage after drying was 21.5 mg/m².

A charge-generation layer was vacuum-deposited onto the adhesive layerby sublimation of the charge-generation material,N,N'-bis(2-phenethyl)perylene-3,4:9,10-bis(dicarboximide), from aresistance-heated tantalum crucible at a temperature of about 181° C., apressure of 1.14×10⁻³ Pa, and a crucible to substrate distance of 25 cm,to achieve a coverage of 380 mg/m².

A charge-transport layer was prepared in darkness by dispersing 0.438gof the charge-transport material,4,4'-bis(diethylamino)tetraphenylmethane, and 70.0 g of the triarylaminecharge-transport material,1,1-bis[4-(di-4-tolylamino)phenyl]-3-phenylpropane, in 1.171 kg of thesolvent, dichloromethane, and then adding to the solvent: 100.8 g ofStructure (I-A) polyester prepared in accordance with Preparations 1 and2, above; 4.2 g of another polymer, poly(ethylene-co-neopentyleneterephthalate) (55:45 molar ratio) (to serve as an adhesion promoter);and 0.33 g of a siloxane surfactant sold under the trademark, DC 510, byDow Corning, U.S.A. The mixture was stirred for 24 hours to dissolve thepolymers in the solvent and was then coated onto the charge-generationlayer and dried to form the charge-transport layer at a dry coverage of23.7 g/m² (a thickness of about 22 micrometers).

The electrophotographic element was then subjected to 50 cycles ofoperation comprising initially uniformly charging the element to -500volts, exposing the element through the CTL to visible actinic radiation(radiation having a peak intensity at a wavelength of 640 nm, to whichthe charge-generation material in the CGL is sensitive in order togenerate electron/hole pairs) up to an amount just sufficient todischarge the element to -100 volts (to simulate imaging exposure), andthen exposing the element to excess visible radiation in order toattempt to erase the remaining charge. The amount of imaging exposure tovisible radiation necessary to reduce the charge from -500 to -100 voltswas only 2.5 ergs/cm² during the initial cycle of operation. After 50cycles of operation, the electrophotographic element was exposed totypical fluorescent room lighting (having typically significant amountsof ultraviolet output) for 15 minutes at an illuminance of 2152 lux, tosimulate adventitious exposure to ultraviolet radiation. The element wasthen subjected to another 50 cycles of operation, and it was found thatthe residual potential remaining in the element after attempted erasureby excess radiation (i.e., after the last step of the last cycle) wasonly -33 volts.

This illustrates that the element exhibited very high speed and littleUV-fogging.

Similar results are achieved when the triarylamine charge-transportmaterial in the CTL is tri-p-tolylamine or1,1-bis[4-(di-p-tolylamino)phenyl]cyclohexane or when the Structure (I)polymer contains recurring units having Structure (I-B) or containsrecurring units having Structure (I-A) and recurring units havingStructure (I-B).

For purposes of comparison, control elements outside the scope of theinvention were also prepared and tested in order to further illustratethe beneficial effects of the invention. The control elements wereprepared and tested exactly as the inventive element described inExample 1, except that the Structure (I) polyester in the CTL wasreplaced with a different polymer (in the same amount) for each controlelement.

In a control element referred to as "Control A", bisphenol Apolycarbonate (sold under the trademark, Makrolon 5705, by MobayChemical Co., U.S.A.) was employed in the CTL instead of the Structure(I) polyester.

In a control element referred to as "Control B", a polycarbonatecomprising recurring units having the structure ##STR4## was employed inthe CTL instead of the Structure (I) polyester.

In a control element referred to as "Control C", polystyrene wasemployed in the CTL instead of the Structure (I) polyester.

                  TABLE I                                                         ______________________________________                                                       residual necessary                                                            potential.sup.1                                                                        exposure.sup.2                                        Example        (volts)  (ergs/cm.sup.2)                                       ______________________________________                                        1               -33     2.5                                                   Control A      -132     2.2                                                   Control B      -226     2.0                                                   Control C       -98     3.9                                                   ______________________________________                                         .sup.1 residual potential, after: 50 cycles of operation, followed by         exposure to ultraviolet radiation, followed by 50 more cycles of operatio     .sup.2 amount of exposure to actinic visible radiation necessary to           discharge element from -500 to -100 volts during initial cycle of             operation                                                                

The results in Table I illustrate that in an element of the invention(Example 1) the UV-fogging problem was greatly minimized, so that theelement remains reusable after UV exposure (residual potential remainsless than -100 volts) in operations involving attempted discharging ofthe element from -500 volts to -100 volts. This is in contrast to thecontrol elements, which exhibited unacceptable residual potential(Control A and B) or borderline residual potential (Control C).

Also, the Example 1 element of the invention exhibited very highsensitivity (the necessary exposure to actinic radiation being onlyabout 14% greater than that required for the Control A element, whichemploys bisphenol A polycarbonate in its CTL). Note further that theControl C element, which was borderline for residual potential,exhibited much lower sensitivity (the necessary exposure to actinicradiation being 77% greater than that required for the Control Aelement).

It should also be noted that the high sensitivity of an element of theinvention containing the Structure (I) polyester was unexpected, giventhat other polyesters known to be useful to avoid UV-fogging, whenemployed as a complete replacement for bisphenol A polycarbonate in atriarylamine-containing CTL, cause electrophotographic elements toexhibit much lower sensitivity. Evidence of this can be found, forexample, in Tables I of U.S. Pat. Nos. 4,840,860 and 4,840,861. Notethat in U.S. Pat. No. 4,840,860, the Example D element, which employed apolyester, formed from 4,4'-(2-norbornylidene) diphenol and terephthalicand azelaic acids, in its CTL, required about 56% more exposure than theExample A element, which employed bisphenol A polycarbonate in its CTL.Note that in U.S. Pat. No. 4,840,861, the Example E element, whichemployed a polyester, formed from 2,2-bis(4-hydroxyphenyl)propane andterephthalic and isophthalic acids, in its CTL, required about 76% moreexposure than the Example A element, which employed bisphenol Apolycarbonate in its CTL.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it should be appreciated thatvariations and modifications can be effected within the spirit and scopeof the invention.

What is claimed is:
 1. In an electrophotographic element comprising:anelectrically conductive support; a charge-generation layer sensitive tovisible or infrared radiation; and a charge-transport layer containing atriarylamine charge-transport material, the improvement wherein thecharge-transport layer comprises a polyester containing recurring unitshaving the structure ##STR5##
 2. The electrophotographic element ofclaim 1, wherein the triarylamine charge-transport material comprises1,1-bis[4-(di-4-tolylamino)phenyl]-3-phenylpropane.
 3. Theelectrophotographic element of claim 1, wherein the polyester containsrecurring units having the structure ##STR6##